Here it was clearly a case of the FedEx flying the approach fast, they even land without full flaps. 28 is a 13,000ft runway, I don't think anything dangerous occurred here.

Of course, but several of those 18 seconds happened with the airplane already overflying the runway, flaring, reducing thrust before touching down.

It's a very high-workload scenario where the green light indication (or lack of) could be overlooked, or if attention is focused on paying attention to the lights to see and verify that it got green, then it's diverted from other critical tasks in this, again, high workload situation.

So you risk missing a lack of green-light and landing with an unsafe gear, or making another mistake from diverting your focus to the landing gear lights.

If the green lights doesn't come on in time, the best-case scenario is a very very low go-around whcih, while it should be safe, it's not nice for a best-case. It's an even higher workload contition and it has its own risks.

And, if nothing of that satisfy you as an explanation, then you must know that Fed-Ex, like most airlines, have a "stabilized approach" criteria in their SOPs that say, among other things, that the airplane must be fully configured for landing (including landing gear down and safe) by a given "gate" altitude that tyically is between 500 and 1000ft. And this airplane was with the gear not-safe way way below 500ft. So, as a minimum, they violated their airline's SOP and hence the federal regulations.